Coated and cured proppants

ABSTRACT

Solid proppants are coated with a coating that exhibits the handling characteristics of a pre-cured coating while also exhibiting the ability to form particle-to-particle bonds at the elevated temperatures and pressures within a wellbore. The coating includes a substantially homogeneous mixture of (i) at least one isocyanate component having at least 2 isocyanate groups, and (ii) a curing agent comprising a monofunctional alcohol, amine or amide. The coating process can be performed with short cycle times, e.g., less than about 4 minutes, and still produce a dry, free-flowing, coated proppant that exhibits low dust characteristics during pneumatic handling but also proppant consolidation downhole for reduced washout and good conductivity. Such proppants also form good unconfined compressive strength without use of an bond activator, are substantially unaffected in bond formation characteristics under downhole conditions despite prior heat exposure, and are resistant to leaching with hot water.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/099,893 entitled “Coated and Cured Proppants” and filed onMay 3, 2011 and also a continuation-in-part of U.S. patent applicationSer. No. 13/188,530 entitled “Coated and Cured Proppants” and filed onJul. 22, 2011. The contents of these co-pending application are herebyincorporated by reference.

FIELD OF INVENTION

The invention relates to a method for the production of coatedproppants, and also to the proppants obtained according to this method,to the uses thereof and to methods which use the proppants.

BACKGROUND OF THE INVENTION

Well fracturing is an often used technique to increase the efficiencyand productivity of oil and gas wells. Overly simplified, the processinvolves the introduction of a fracturing fluid into the well and theuse of fluid pressure to fracture and crack the well strata. The cracksallow the oil and gas to flow more freely from the strata and therebyincrease production rates in an efficient manner.

There are many detailed techniques involved in well fracturing, but oneof the most important is the use of a solid “proppant” to keep thestrata cracks open as oil, gas, water and other fluids found in wellflow through those cracks. The proppant is carried into the well withthe fracturing fluid which itself may contain a variety of viscosityenhancers, gelation agents, surfactants, etc. These additives alsoenhance the ability of the fracturing fluid to carry proppant to thedesired strata depth and location. The fracturing fluid for a particularwell may or may not use the same formulation for each depth in thestrata.

Proppants can be made of virtually any generally solid particle that hasa sufficiently high crush strength to prop open cracks in a rock strataat great depth and temperatures of about 35° C. and higher. Sand andceramic proppants have proved to be especially suitable for commercialuse.

A proppant that is flushed from the well is said to have a high “flowback.” Flow back is undesirable. In addition to closure of the cracks,the flushed proppants are abrasive and can damage or clog the tubulargoods used to complete the well, valves and pipelines in downstreamprocessing facilities.

One type of synthetic resin coatings can be used to impart a degree ofadhesion to the proppant so that flow back is substantially reduced oreliminated. Such resins can include phenol resin, epoxy resin,polyurethane-phenol resin, furane resin, etc. See published US PatentApplication Nos. 2002/0048676, 2003/0131998, 2003/0224165, 2005/0019574,2007/0161515 and 2008/0230223 as well as U.S. Pat. Nos. 4,920,192;5,048,608; 5,199,491; 6,406,789; 6,632,527; 7,624,802; and publishedinternational application WO 2010/049467, the disclosures of which areherein incorporated by reference.

With some coatings, the synthetic coating is not completely cured whenthe proppant is introduced into the well. The coated, partially-curedproppants are free flowing, but the coating resin is still slightlyreactive. The final cure is intended to occur in situ in the stratafracture at the elevated closure pressures and temperatures found “downhole.”

Such partially cured coating can also exhibit a number of performanceissues ranging from:

-   -   A lack of storage stability if stored in a hot environment. This        type situation could result in a completion of the curing        process while in storage making the coated proppant incapable of        bonding when placed in the fracture.    -   Leaching chemicals out of the partially cured coating that could        interfere with the viscosity profile of the fluid used to carry        the proppant into the fracture or the chemical breaker system        that is relied on to reduce the frac fluid viscosity after        completion of the fracturing treatment.    -   Erosion of the partially cured coating when the coated proppant        is handled pneumatically in order to place in the field storage        bins at the well site.    -   Premature curing in the fracture due to extended exposure to the        elevated temperatures found downhole but before the cracks heal        and begin to force the proppant grains into contact with each        other.

A second type of synthetic coating is described as being pre-cured ortempered. In this case the coating is essentially cured during themanufacturing process. This type of coating will strengthen thesubstrate particle so that it can withstand a higher stress level beforegrain failure. Such a pre-cured coating with also exhibit the followingtraits: (1) Excellent storage stability; (2) Minimal chemicals that canbe leached out of the coating to interfere with carrier fluid viscosityor breaker systems; and (3) A coating that is resilient to the abrasionof pneumatic handling.

The main limitation of a pre-cured coating is that it cannot createsignificant particle to particle bonding when placed in the fracture andtemperature and closure pressure are applied. This means that apre-cured coated particle can do little to prevent proppant flowbackafter the well is opened up to start the clean-up process or to producethe well. Such pre-cured products can also exhibit reductions in bondingcapability and/or strength if exposed to elevated temperatures duringhandling or storage.

Proppants based on polyurethane chemistries have a number of potentialadvantages over phenol resin systems. Most notably, the reaction ratesused to make polyurethane coatings are generally faster than phenolresins, cure at lower temperatures and do not have gaseous emissionsthat require specialized recovery equipment. The coating step withpolyurethanes can be carried out at temperatures of about 10° C. toabout 250° C. although temperatures of less than about 110° C. arepreferred to minimize emissions during the coating process as well asenergy use. Polyurethane coatings can also be performed without the useof solvents, whereas many of the known methods, as a rule, requireorganic solvents for the resinous coating. The components inpolyurethane systems are also generally easier to use and pose lowerenvironmental issues. These factors could reduce the cost to make coatedproppants.

Previously described polyurethanes have not, however, achievedwidespread adoption due to their performance in the hot, wet, highpressured environment encountered in the fracture. The stability of thecoating to this environment, the ability of the coating to preventparticle failure (e.g., by crushing) and to develop strongparticle-to-particle bonds, have contributed to poor flowback controland less than desirable fracture conductivity.

Low temperature wells pose certain problems for coated proppants. Priorto the present invention, the only option that was available to the oiland gas well industry for controlling flowback in a well having a lowformation temperature, e.g., <140° F. (60° C.) was to use a partiallycured, phenolic-coated proppant in combination with a type ofplasticizer called a bond “activator”. Without the bond activator, thephenolic coating cured too slowly to generate sufficient bond strengthin a reasonable amount of time. The activator plasticizer softens thecoating so that the coating can gain some adhesion properties when thecoated proppant solids are pushed into contact due to the closure stressfrom the closing of the fractured strata cracks. This adhesion willnever result in a substantial measurable unconfined compressive strengthbut can result in a somewhat consolidated sample. The activator would beused at concentrations ranging from 5-20 gallons/1000 gallons offracturing fluid (known as “frac” fluid). While the activator can helpthe phenolic coated proppant to function (to some degree) in lowtemperature applications it does have the following issues:

-   -   The activator loadings add substantial cost to the treatment.    -   The activator chemistry can create problems with frac fluid        rheology and breakers systems.    -   The use of an activator does not result in a significant        measureable particle to particle bond strength.    -   The activator is another factor in trying to quantify the        effects of a fracturing treatment on the environment.    -   The phenolic coating also has environmental issues because of        the components that can be leached out of the coating        (formaldehyde, phenol and hexamethylenetetramine)

SUMMARY OF THE INVENTION

It would be desirable to develop a coated proppant that combinedadequate crush resistance, resistance to dusting when handledpneumatically and the limited leaching of chemicals that is exhibited bypre-cured coatings with the ability to create the particle-to-particlebonds that resist proppant flowback which are exhibited by a partiallycured proppant coating. Even more desirably, this bonding ability isrelatively unaffected by prolonged exposure to elevated temperatures inthe fracture so that the coated proppants continue to bond as stress isapplied when the fracture heals and forces the coated proppantstogether.

It would also be desirable to have a coated proppant that retained itsconductivity under the conditions prevailing within an activelyproducing well field stratum.

It would further be desirable to have a coating that not only exhibitedall of these properties but which had desirable fracture conductivityand a short production cycle.

It would be especially preferred if a proppant could be substantiallycovered with a fast cure coating that could be produced to a freeflowing state in a short period, e.g., less than about four minutes,while also exhibiting good crush resistance, resistance to hot watercoating loss and low dust generation during pneumatic conveyance.

It would also be desirable to have a coating that would not require theuse of an activator in order to generate measurable bond strength at lowtemperature, that would eliminate the need for an added activator whichmight affect interactions between the frac fluid and a breaker, andavoid environmental effects from the use of an activator and componentsthat can come out of a phenolic coating on the proppant solids.

These and other objectives of the invention that will become apparentfrom the description herein can be accomplished by a coating and coatingprocess that comprises the step of: coating a proppant solid with asubstantially homogeneous coating mixture that comprises (i) at leastone isocyanate-functional component having at least 2 isocyanate groups,and (ii) at least one curing agent comprising a monofunctional alcohol,amine or amide, in an amount sufficient and under conditions sufficientto substantially cure said proppant coating and form free flowing,coated proppants in a period of time of less than about four minutes toform a free-flowing, substantially cured, coated proppant.

A coated, free flowing proppant according to the invention comprises asolid proppant core particle that is substantially covered with acoating that comprises the reaction product of a coating mixture thatcomprises at least one isocyanate component and a curing agent to form asubstantially fully cured proppant coating that is capable of formingparticle-to-particle bonds at elevated temperature and pressure, such asthose found downhole in an oil or gas well.

The coating process of the present invention results one or more layersof cured polyurethane around a solid proppant core that is substantiallycured and crosslinked quickly to produce a coated proppant product thatacts like it has a hybrid coating, i.e., one that acts like a pre-curedcoating in its resists dissolution of the coating under the rigorouscombination of high heat, agitation, abrasion and water found downholein an oil or gas well; exhibits good crush resistance and fractureconductivity; and has a tough coating that exhibits low levels of dustor fines generation during pneumatic conveyance as well as in usedownhole but also exhibits traits of a partially cured coating in itsability to form particle-particle bonds with similarly coated proppantsat downhole conditions. In addition, the coating process has a highproduction rate due to its low cycle time for the coating/curingprocess, low emission level and a low overall production cost and doesnot lose bonding capability if exposed to elevated temperatures duringhandling or storage.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a TMA plot of Dimension Change at various Temperatures in aTMA test of pre-cured, partially cured Phenolic A, partially curedPhenolic B that is more curable than Phenolic A, and the coating of thepresent invention as discussed in Example 6.

FIG. 2 is a bar chart of crush test results of various coated proppantstested in Example 6.

FIG. 3 is a chart of Unconfined Compressive Strength for the coatedproppants of Example 6.

FIG. 4 charts the fracture conductivity of three coated proppants usedin Example 6.

FIG. 5 depicts the results of coating loss tests in simulated welldown-hole conditions.

FIGS. 6 and 7 illustrate the results of comparative conductivity testsin a simulated low temperature well using a proppant according to thepresent invention and a prior art proppant coated with a phenolic resin.

FIG. 8 shows the results of high temperature tests for unconfinedcompressive strength under high temperature well conditions and eitherwith or without a three hour preheat exposure.

DETAILED DESCRIPTION OF THE INVENTION

The coating formulation of the present invention includes asubstantially homogeneous mixture of a curable coating formulation thatcomprises: (a) at least one isocyanate-functional reactant having atleast 2 isocyanate groups, and (ii) at least one curing agent comprisinga monofunctional alcohol, amine or amide. The coating formulation mayfurther comprise one or more curing agents in the form of aminereactants, metal catalysts, and/or polyol-functional reactants. Thecomponents are used in an amount sufficient and under conditions thatare also sufficient to substantially cure the proppant coating and formfree flowing, coated proppants in a fairly short period of time. Thecoated proppant thus exhibits the good handling and low dustcharacteristics of a pre-cured product but also exhibits in-strataconsolidation characteristics and flow-back resistance that are like apartially cured product.

The coating process of the present invention applies one or more layersof a curable coating formulation around a solid proppant core that isquickly and substantially cured to resist dissolution of the coatingunder the rigorous combination of high heat, agitation, abrasion andwater that are found downhole in a well. Preferably, the cured coatingexhibits a sufficient resistance to a 10 day autoclave test or 10 dayconductivity test so that the coating resists loss by dissolution in hotwater of less than 25 wt %, more preferably less than 15 wt %, and evenmore preferably a loss of less than 5 wt %. The substantially curedcoating of the invention thus resists dissolution in the fracturedstratum while also exhibiting sufficient particle-to-particle reactionbond strength to resist flow back and sufficiently high crush strengthto maintain conductivity of the fractures.

A preferred testing method for the effectiveness of a proppant isdescribed in ISO 13503-5:2006(E) “Procedures for measuring the long termconductivity of proppants”, the disclosure of which is hereinincorporated by reference. ISO 13503-5:2006 provides standard testingprocedures for evaluating proppants used in hydraulic fracturing andgravel packing operations. ISO 13503-5:2006 provides a consistentmethodology for testing performed on hydraulic fracturing and/or gravelpacking proppants. The “proppants” mentioned henceforth in this part ofISO 13503-5:2006 refer to sand, ceramic media, resin-coated proppants,gravel packing media, and other materials used for hydraulic fracturingand gravel-packing operations. ISO 13503-5:2006 is not applicable foruse in obtaining absolute values of proppant pack conductivities underdownhole reservoir conditions, but it does serve as a consistent methodby which such downhole conditions can be simulated and compared in alaboratory setting.

The present invention is particularly directed to a proppant coatingtechnology that exhibits decidedly different traits when kept dry (as ina storage bin) as opposed to when the coated proppant is added to thefrac fluid and pumped into the fracture. One way to characterize thisdifference is by analyzing the test results from a TMA (thermalmechanical analyzer). In a dry state, the preferred coating will show aTg softening point that is well above any possible storage temperature(>75° C.). This assures that the coated product can be safely stored aslong as it is kept relatively dry. However, when water is added to theTMA test sample, the resulting Tg is now measured at a level that is<50° C. At this Tg, the coated sand will have the necessary propertiesto promote adhesion at low temperature applications once the fracturehas closed and the resulting differential stress is placed on theproppants. Since the coating's ability to bond is not related to achemical reaction rate, once the fracture has closed thereby exerting aclosure stress on the proppant pack, there is no need for an extendedshut-in period, e.g., 18-24 hours, before opening the well up to cleanupand production.

The Isocyanate Component

The isocyanate-functional component for the present invention comprisesan isocyanate-functional component with at least 2 reactive isocyanategroups. Other isocyanate-containing compounds may be used, if desired.Examples of suitable isocyanate with at least 2 isocyanate groups analiphatic or an aromatic isocyanate with at least 2 isocyanate groups(e.g. a diisocyanate, triisocyanate or tetraisocyanate), or an oligomeror a polymer thereof can preferably be used. These isocyanates with atleast 2 isocyanate groups can also be carbocyclic or heterocyclic and/orcontain one or more heterocyclic groups.

The isocyanate-functional component with at least 2 isocyanate groups ispreferably a compound or oligomer of compounds of the formula (III) or acompound of the formula (IV):

In the formulas (III) and (IV), A is each, independently, an aryl,heteroaryl, cycloalkyl or heterocycloalkyl. Preferably, A is each,independently, an aryl or cycloalkyl. More preferably A is each,independently, an aryl which is preferably phenyl, naphthyl oranthracenyl, and most preferably phenyl. Still more preferably A is aphenyl.

The above mentioned heteroaryl is preferably a heteroaryl with 5 or 6ring atoms, of which 1, 2 or 3 ring atoms are each, independently, anoxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heteroaryl is selected among pyridinyl,thienyl, furyl, pyrrolyl, imidazolyl, pyrazolyl, pyrazinyl, pyrimidinyl,pyridazinyl, oxazolyl, isoxazolyl or furazanyl.

The above mentioned cycloalkyl is preferably a C₃₋₁₀-cycloalkyl, morepreferably a C₅₋₇-cycloalkyl.

The above mentioned heterocycloalkyl is preferably a heterocycloalkylwith 3 to 10 ring atoms (more preferably with 5 to 7 ring atoms), ofwhich one or more (e.g. 1, 2 or 3) ring atoms are each, independently,an oxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heterocycloalkyl is selected from amongtetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, acetidinyl,pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl,oxazolidinyl or isoxazolidinyl. Still more preferably, theheterocycloalkyl is selected from among tetrahydrofuranyl, piperidinyl,piperazinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, oxazolidinyl or isoxazolidinyl.

In the formulas (III) and (IV), each R¹ is, independently, a covalentbond or C₁₋₄-alkylene (e.g. methylene, ethylene, propylene or butylene).Preferably each R² is hydrogen or a covalent bond.

In the formulas (III) and (IV), each R² is each, independently,hydrogen, a halogen (e.g. F, Cl, Br or I), a C₁₋₄-alkyl (e.g. methyl,ethyl, propyl or butyl) or C₁₋₄-alkyoxy (e.g. methoxy, ethoxy, propoxyor butoxy). Preferably, each R² is, independently, hydrogen or aC₁₋₄-alkyl. More preferably each R² is hydrogen or methyl.

In the formula (IV), R³ is a covalent bond, a C₁₋₄-alkylene (e.g.methylene, ethylene, propylene or butylene) or a group—(CH₂)_(R31)—O—(CH₂)_(R32)—, wherein R31 and R32 are each,independently, 0, 1, 2 or 3. Preferably, R³ is a —CH₂— group or an —O—group.

In the formula (III), p is equal to 2, 3 or 4, preferably 2 or 3, morepreferably 2.

In the formulas (III) and (IV), each q is, independently, an integerfrom 0 to 4, preferably 0, 1 or 2. When q is equal to 0, thecorresponding group A has no substituent R², but has hydrogen atomsinstead of R².

In the formula (IV), each r and s are, independently, 0, 1, 2, 3 or 4,wherein the sum of r and s is equal to 2, 3 or 4. Preferably, each r ands are, independently, 0, 1 or 2, wherein the sum of r and s is equal to2. More preferably, r is equal to 1 and s is equal to 1.

Examples of the isocyanate with at least 2 isocyanate groups are:toluol-2,4-diisocyanate; toluol-2,6-diisocyanate;1,5-naphthalindiisocyanate; cumol-2,4-diisocyanate;4-methoxy-1,3-phenyldiisocyanate; 4-chloro-1,3-phenyldiisocyanate;diphenylmethane-4,4-diisocyanate; diphenylmethane-2,4-diisocyanate;diphenylmethane-2,2-diisocyanate; 4-bromo-1,3-phenyldiisocyanate;4-ethoxy-1,3-phenyl-diisocyanate; 2,4′-diisocyanate diphenylether;5,6-dimethyl-1,3-phenyl-diisocyanate; methylenediphenyl diisocyanate(including 2,2′-MDI, 2,4′-MDI and 4,4″-MDI);4,4-diisocyanato-diphenylether; 4,6-dimethyl-1,3-phenyldiisocyanate;9,10-anthracene-diisocyanate; 2,4,6-toluol triisocyanate;2,4,4′-triisocyanatodiphenylether; 1,4-tetramethylene diisocyanate;1,6-hexamethylene diisocyanate; 1,10-decamethylene-diisocyanate;1,3-cyclohexylene diisocyanate;4,4′-methylene-bis-(cyclohexylisocyanate); xylol diisocyanate;1-isocyanato-3-methyl-isocyanate-3,5,5-trimethylcyclohexane (isophoronediisocyanate); 1-3-bis(isocyanato-1-methylethyl) benzol (m-TMXDI);1,4-bis(isocyanato-1-methylethyl) benzol (p-TMXDI); oligomers orpolymers of the above mentioned isocyanate compounds; or mixtures of twoor more of the above mentioned isocyanate compounds or oligomers orpolymers thereof.

Particularly preferred isocyanates with at least 2 isocyanate groups aretoluol diisocyanate, methylenediphenyl diisocyanate, diphenylmethanediisocyanate, an oligomer based on toluol diisocyanate, an oligomerbased on methylenediphenyl diisocyanate (poly-MDI) or an oligomer basedon diphenylmethane diisocyanate and polymers thereof.

Curing Agents

The coatings of the invention can be cured with at least one of avariety of curing agents, including reactive, non-reactive (e.g.,“catalysts”) and partially reactive agents. Generally, preferred curingagents are selected from amine-based curing agents, hydroxyl-functionalcuring agents, polyols, and/or metal-based catalysts. Particularlypreferred curing agents are one or more monofunctional alcohols, aminesand/or amides. The amine-based curing agents may also be used as amixture of a fast-acting first curing agent and a second, latent curingagent. Either of these first and/or second amine-based curing agents maybe reactive, nonreactive or partially reactive.

Suitable single amine-based curing agent or a mixture of amine-basedcuring agents can include, but are not limited to, ethylene diamine;hexamethylene diamine; 1-methyl-2,6-cyclohexyl diamine; 2,2,4- and2,4,4-trimethyl-1,6-hexanediamine;4,4′-bis-(sec-butylamino)-dicyclohexylmethane and derivatives thereof;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino)-cyclohexane; 4,4′-dicyclohexylmethane diamine;1,4-cyclohexane-bis-(methylamine); 1,3-cyclohexane-bis-(methylamine),isomers, and mixtures thereof; diethylene glycol bis-(aminopropyl)ether;2-methylpentamethylene-diamine; diaminocyclohexane, isomers, andmixtures thereof; diethylene triamine; triethylene tetramine;tetraethylene pentamine; propylene diamine; 1,3-diaminopropane;dimethylamino propylamine; diethylamino propylamine;imido-bis-(propylamine); monoethanolamine, diethanolamine;triethanolamine; monoisopropanolamine, diisopropanolamine;isophoronediamine; 4,4′-methylenebis-(2-chloroaniline);3,5-dimethylthio-2,4-toluenediamine;3,5-dimethylthio-2,6-toluenediamine; 3,5-diethylthio-2,4-toluenediamine;3,5-diethylthio-2,6-toluenediamine; 4,4′-bis-(sec-butylamino)-benzene;and derivatives thereof; 1,4-bis-(sec-butylamino)-benzene;1,2-bis-(sec-butylamino)-benzene; N,N′-dialkylamino-diphenylmethane;trimethyleneglycol-ci-p-aminobenzoate;polytetramethyleneoxide-di-p-aminobenzoate;4,4′-methylenebis-(3-chloro-2,6-diethyleneaniline);4,4′-methylenebis-(2,6-diethylaniline); meta-phenylenediamine;paraphenylenediamine; N,N′-diisopropyl-isophoronediamine;polyoxypropylene diamine; propylene oxide-based triamine;3,3′-dimethyl-4,4′-ciaminocyclohexylmethane; and mixtures thereof. Inone embodiment, the amine-terminated curing agent is4,4′-bis-(sec-butylamino)-dicyclohexylmethane. Preferred amine-basedcuring agents for use with the present invention includetriethylenediamine; bis(2-dimethylaminoethyl)ether;tetramethylethylenediamine; pentamethyldiethylenetriamine;1,3,5-tris(3-(dimethylamino)propyl)-hexahydro-s-triazine and othertertiary amine products of alkyleneamines.

Additionally, other catalysts that promote the reaction of isocyanateswith hydroxyls and amines that are known by the industry can be used inthe present invention, e.g., transition metal co-catalysts of Groups IIIor IV used for polyurethane foams. A particularly preferred metalco-catalyst is a tin complex such as stannous 2-ethylhexanoate or anorganotin compound, such as dibutyltin dilaurate and tin-containingsalts.

Also preferred are catalysts that promote isocyanate trimerization overother reaction mechanisms. See, e.g., U.S. Pat. No. 5,264,572 (cesiumfluoride or tetraalkylammonium fluoride), U.S. Pat. No. 3,817,939(organic carbonate salt), and U.S. Pat. No. 6,127,308 (lithium salts,lithium hydroxide, allophane catalysts such as tin-2-ethylhexanoate ortin octoate, and organic compounds containing at least one hydroxylgroup), the disclosures of which are herein incorporated by reference.

The amine-based curing agent may have a molecular weight of about 64 orgreater. In one embodiment, the molecular weight of the amine-curingagent is about 2000 or less. In addition, any of the amine-terminatedmoieties listed above for use as the isocyanate-reactive component toform the prepolymer may be used as curing agents to react with theprepolymers.

Of the list above, the saturated amine-based curing agents suitable foruse with the present invention include, but are not limited to, ethylenediamine; hexamethylene diamine; 1-methyl-2,6-cyclohexyl diamine; 2,2,4-and 2,4,4-trimethyl-1,6-hexanediamine;4,4′-bis-(sec-butylamino)-dicyclohexylmethane;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino-cyclohexane; derivatives of4,4′-bis-(sec-butylamino)-dicyclohexylmethane; 4,4′-dicyclohexylmethanediamine; 1,4-cyclohexane-bis-(methylamine);1,3-cyclohexane-bis-(methylamine); diethylene glycolbis-(aminopropyl)ether; 2-methylpentamethylene-diamine;diaminocyclohexane; diethylene triamine; triethylene tetramine;tetraethylene pentamine; propylene diamine; dipropylene triamine;1,3-diaminopropane; dimethylamino propylamine; diethylamino propylamine;imido-bis-(propylamine); monoethanolamine, diethanolamine;triethanolamine; monoisopropanolamine, diisopropanolamine;triisopropanolamine; isophoronediamine; N,N′-diisopropylisophoronediamine and mixtures thereof.

In one embodiment, the curative used with the prepolymer include3,5-dimethylthio-2,4-toluenediamine,3,5-dimethyl-thio-2,6-toluenediamine,4,4′-bis-(sec-butylamino)-diphenylmethane, N,N′-diisopropyl-isophoronediamine; polyoxypropylene diamine; propylene oxide-based triamine;3,3′-dimethyl-4,4′-diaminocyclohexylmethane; and mixtures thereof.

Because unhindered primary diamines result in a rapid reaction betweenthe isocyanate groups and the amine groups, in certain instances, ahindered secondary diamine may be more suitable for use in theprepolymer. Without being bound to any particular theory, it is believedthat an amine with a high level of stearic hindrance, e.g., a tertiarybutyl group on the nitrogen atom, has a slower reaction rate than anamine with no hindrance or a low level of hindrance. For example,4,4′-bis-(sec-butylamino)-dicyclohexylmethane (CLEARLINK 1000® fromHuntsman Corporation in The Woodlands, Tex.) may be suitable for use incombination with an isocyanate to form the polyurea prepolymer. Inaddition, N,N′-diisopropyl-isophorone diamine, also available fromHuntsman Corporation, under the tradename JEFFLINK®, may be used as thesecondary diamine curing agent.

In addition, a trifunctional curing agent can be used to help improvecross-linking and, thus, to further improve the chemical and/or abrasionresistance of the coating. In one embodiment, a triol such astrimethylolpropane or a tetraol such as N,N,N′,N′-tetrakis(2-hydroxylpropyl)ethylenediamine may be added to the formulations.

The curing agents of the present invention can be added to the coatingformulation with the polyol component, the amine-reactive polyolcomponent, any of the additives (e.g., coloring agents) or addedsimultaneously as any of these components or pre-coated on the proppant.Preferably, the curing agent is mixed with or co-applied to the solidproppant core as the first isocyanate and any other reactants are mixedso that the curing process has begun by the time the coating formulationis applied to the surface of the solid proppant core. It is alsopossible to premix the isocyanate and polyol together immediately beforeentry into the mixer. This probably would give a slightly more uniformdistribution of the chemicals in the coating. Alternately, it would bepossible to premix the polyol and curing agent before they are added tothe isocyanate.

Most preferably, the isocyanate, polyol, combination of (a) polyol and(b) curing agent or each individually are continuously added to solidproppant in a moving mixer at a rate that is not substantially greaterthan the rate of the crosslinking reaction between and among theingredients. The specific rate will depend on the size of the mixer, thetype of mixer, and whether batch or continuous production is desired.The goal is to substantially completely coat the proppant solid with acoating that becomes cured in the mixer and is discharged as afree-flowing, discrete particulates. The amperage draw rate of the mixercan be used as a guide in tumbling-type mixers because the build-up ofan uncured, tacky coating on the proppant solids will increase the loadon the mixer motor which can be monitored by a simple amp meter. Addingthe reaction components at a rate that is consistent with the reactionrate of the curing process avoids substantial increases in amperageallows the coating process, avoids stalling the motor or interruptingthe coating process, and maximizes the productivity of the equipmentused to perform the coating/curing process. In a preferred process usinglaboratory scale equipment, a few seconds after beginning to add thepolyol, the isocyanate is added at a controlled rate over a relativelyshort period, e.g., about a minute.

Hydroxy-Functional Curing Agents

The proppant coating of the invention may also be cured alone or withother curing agents with a single hydroxy-terminated curing agent (i.e.,a monol such as C1-C20 alcohols such as ethanol, isopropyl, butanol, orstearyl alcohol) or a mixture of hydroxy-terminated curing agents. Theappropriate use of such a monol capping agent or chain terminator canhelp to control the impact of internal, unreacted —NCO groups that canhave adverse properties on the final coating. Indeed, the use of monolwithin the range from about 1 equivalent wt % to about 30 equivalent wt% relative to the weight of any added polyhydroxy compounds can bringinto the coating certain properties that are not related to the originalisocyanate or polyhydroxy component, such as enhanced or decreasedhydrophobicity, corrosion resistance, viscosity modification infracturing fluid, reduce the frictional drag of production fluids oncein the fracture, ion exchange and antimicrobial effects.

Suitable hydroxy-terminated curing agents include, but are not limitedto, ethanol, ethylene glycol; diethylene glycol; polyethylene glycol;propylene glycol; 2-methyl-1,3-propanediol; 2,-methyl-1,4-butanediol;dipropylene glycol; polypropylene glycol; 1,2-butanediol;1,3-butanediol; 1,4-butanediol; 2,3-butanediol;2,3-dimethyl-2,3-butanediol; trimethylolpropane; cyclohexyldimethylol;triisopropanolamine; N,N,N′N′-tetra-(2-hydroxypropyl)-ethylene diamine;diethylene glycol bis-(aminopropyl)ether; 1,5-pentanediol;1,6-hexanediol; 1,3-bis-(2-hydroxyethoxy)cyclohexane;1,4-cyclohexyldimethylol;1,3-bis-[2-(2-hydroxyethoxy)ethoxy]cyclohexane;1,3-bis-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}cyclohexane;polytetramethylene ether glycol, preferably having a molecular weightranging from about 250 to about 3900;resorcinol-di-(beta-hydroxyethyl)ether and its derivatives;hydroquinone-di-(beta-hydroxyethyl)ether and its derivatives;1,3-bis-(2-hydroxyethoxy)benzene;1,3-bis-[2-(2-hydroxyethoxy)ethoxy]benzene;1,3-bis-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}benzene;N,N-bis(.beta.-hydroxypropyl)aniline; 2-propanol-1,1′-phenylaminobis;and mixtures thereof.

The hydroxy-terminated curing agent may have a molecular weight of atleast about 50. In one embodiment, the molecular weight of thehydroxy-terminated curing agent is about 2000 or less. In yet anotherembodiment, the hydroxy-terminated curing agent has a molecular weightof about 250 to about 3900. It should be understood that molecularweight, as used herein, is the absolute weight average molecular weightand would be understood as such by one of ordinary skill in the art.

The saturated hydroxy-terminated curing agents, included in the listabove, are preferred when making a light stable composition. Thosesaturated hydroxy-terminated curing agents include, but are not limitedto, ethylene glycol; diethylene glycol; polyethylene glycol; propyleneglycol; 2-methyl-1,3-propanediol; 2,-methyl-1,4-butanediol; dipropyleneglycol; polypropylene glycol; 1,2-butanediol; 1,3-butanediol;1,4-butanediol; 2,3-butanediol; 2,3-dimethyl-2,3-butanediol;trimethylolpropane; cyclohexyldimethylol; triisopropanolamine;N,N,N′,N′-tetra-(2-hydroxypropyl)-ethylene diamine; diethylene glycolbis-(aminopropyl)ether; 1,5-pentanediol; 1,6-hexanediol;1,3-bis-(2-hydroxyethoxy)cyclohexane; 1,4-cyclohexyldimethylol;1,3-bis-[2-(2-hydroxyethoxy)ethoxy]cyclohexane;1,3-bis-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}cyclohexane;polytetramethylene ether glycol having molecular weight ranging fromabout 250 to about 3900; and mixtures thereof.

The amount of curing agent that is added to the coating will generallyfall within the range from about 0.01 wt % to about 95 wt % of thecomplete coating formulation.

Polyol Curing Agents

A polyol component can be added to the coating formulation. The polyolcomponent may or may not have reactive amine functionality and cancomprise oxides, polyesters, polyamides, polyurethane, epoxy, siliconeor polysiloxane, or vinyl backbones which react to become an integralpart of the resulting coating on the proppant core.

A useful polyurethane coating is a phenolic polyurethane made with aphenolic polyol according to a patent application that was filed withthe German Patent Office under no. DE 10 2010 051 817.4 on Nov. 19, 2010and entitled “Proppant Coating Technology”, the disclosure of which isherein incorporated by reference and summarized below in the context ofthe process of the present invention.

Another polyol component for the present process comprises a phenolresin that comprises a condensation product of a phenol and an aldehyde,such as formaldehyde. The phenol resin is preferably a resole or novolakphenol resin and more preferably a benzyl ether resin.

The resole-type phenol resin can be obtained, for example, bycondensation of phenol or of one or more compounds of the followingformula (I), with aldehydes, preferably formaldehyde, under basicconditions.

In the formula (I):

-   -   “R” is in each case, independently, a hydrogen atom, a halogen        atom, C₁₋₁₆-alkyl (preferably C₁₋₁₂-alkyl, more preferably        C₁₋₆-alkyl, and still more preferably methyl, ethyl, propyl or        butyl) or —OH;    -   “p” is an integer from 0 to 4, preferably 0, 1, 2 or 3, and more        preferably 1 or 2. Those in the art will understand that when p        is 0, the compound of formula (I) is phenol.

Novolak-type phenol resin for the present invention comprises thecondensation product of phenol or of one or more compounds of theformula (I) defined above, with aldehydes, preferably formaldehyde,under acidic conditions.

In another preferred embodiment, the phenol resin is a benzyl etherresin of the general formula (II):

In the formula (II):

-   -   A, B and D each are, independently, a hydrogen atom, a halogen        atom, a C₁₋₁₆-hydrocarbon residue, —(C₁₋₁₆-alkylene)-OH, —OH, an        —O—(C₁₋₁₆-hydrocarbon residue), phenyl, —(C₁₋₆-alkylene)-phenyl,        or —(C₁₋₆-alkylene)-phenylene-OH;    -   The halogen atom is F, Cl, Br or I;    -   The C₁₋₁₆-hydrocarbon-residue is preferably C₁₋₁₆-alkyl,        C₂₋₁₆-alkenyl or C₂₋₁₆-alkinyl, more preferably C₁₋₁₂-alkyl,        C₂₋₁₂-alkenyl or C₂₋₁₂-alkinyl, still more preferably        C₁₋₆-alkyl, C₂₋₆-alkenyl or C₂₋₆-alkinyl, and still more        preferably C₁₋₄-alkyl, C₂₋₄-alkenyl or C₂₋₄-alkinyl, and still        more preferably C₁₋₁₂-alkyl, and still more preferably        C₁₋₆-alkyl, and still more preferably methyl, ethyl, propyl or        butyl, and most preferably methyl;    -   The residue —(C₁₋₁₆-alkylene)-OH is preferably        —(C₁₋₁₂-alkylene)-OH, more preferably —(C₁₋₆-alkylene)-OH, and        still more preferably —(C₁₋₄-alkylene)-OH, and most preferably a        methylol group (—CH₂—OH);    -   The —O—(C₁₋₁₆-hydrocarbon)-residue is preferably C₁₋₁₆-alkoxy,        more preferably C₁₋₁₂-alkoxy, and still more preferably        C₁₋₆-alkoxy, and still more preferably C₁₋₄-alkoxy, and still        more preferably —O—CH₃, —O—CH₂CH₃, —O—(CH₂)₂CH₃ or —O—(CH₂)₃CH₃;    -   The residue —(C₁₋₆-alkylene)-phenyl is preferably        —(C₁₋₄-alkylene)-phenyl, and more preferably —CH₂-phenyl;    -   The residue —(C₁₋₆-alkylene)-phenylene-OH is preferably        —(C₁₋₄-alkylene)-phenylene-OH, and more preferably        —CH₂-phenylene-OH;    -   R is a hydrogen atom of a C₁₋₆-hydrocarbon residue (e.g. linear        or branched C₁₋₆-alkyl). R is particularly preferred as a        hydrogen atom. This is the case, for example, when formaldehyde        is used as aldehyde component in a condensation reaction with        phenols in order to produce the benzyl ether resin of the        formula (II);    -   m¹ and m² are each, independently, 0 or 1.    -   n is an integer from 0 to 100, preferably an integer from 1 to        50, more preferably from 2 to 10, and still more preferably from        2 to 5; and    -   wherein the sum of n, m¹ and m² is at least 2.

In a still further embodiment, the polyol component is a phenol resinwith monomer units based on cardol and/or cardanol. Cardol and cardanolare produced from cashew nut oil which is obtained from the seeds of thecashew nut tree. Cashew nut oil consists of about 90% anacardic acid andabout 10% cardol. By heat treatment in an acid environment, a mixture ofcardol and cardanol is obtained by decarboxylation of the anacardicacid. Cardol and cardanol have the structures shown below:

As shown in the illustration above, the hydrocarbon residue(—C₁₅H_(31-n)) in cardol and/or in cardanol can have one (n=2), two(n=4) or three (n=6) double bonds. Cardol specifically refers tocompound CAS-No. 57486-25-6 and cardanol specifically to compoundCAS-No. 37330-39-5.

Cardol and cardanol can each be used alone or at any particular mixingratio in the phenol resin. Decarboxylated cashew nut oil can also beused.

Cardol and/or cardanol can be condensed into the above described phenolresins, for example, into the resole- or novolak-type phenol resins. Forthis purpose, cardol and/or cardanol can be condensed e.g. with phenolor with one or more of the above defined compounds of the formula (I),and also with aldehydes, preferably formaldehyde.

The amount of cardol and/or cardanol which is condensed in the phenolresin is not particularly restricted and preferably is from about 1 wt %to about 99 wt %, more preferably about 5 wt % to about 60 wt %, andstill more preferably about 10 wt % to about 30 wt %, relative to 100 wt% of the amount of phenolic starting products used in the phenol resin.

In another embodiment, the polyol component is a phenol resin obtainedby condensation of cardol and/or cardanol with aldehydes, preferablyformaldehyde.

A phenol resin which contains monomer units based on cardol and/orcardanol as described above, or which can be obtained by condensation ofcardol and/or cardanol with aldehydes, has a particularly low viscosityand can thus preferably be employed with a low addition or withoutaddition of reactive thinners. Moreover, this kind of long-chain,substituted phenol resin is comparatively hydrophobic, which results ina favorable shelf life of the coated proppants obtained by the methodaccording to the present invention. In addition, a phenol resin of thiskind is also advantageous because cardol and cardanol are renewable rawmaterials.

Apart from the phenol resin, the polyol component can still containother compounds containing hydroxyl groups. The other compoundscontaining hydroxyl groups can be selected from the compounds containinghydroxyl groups that are known to be useful for making polyurethanes,e.g., hydroxy-functional polyethers, hydroxy-functional polyesters,alcohols or glycols. One preferred compound containing hydroxyl groupsis, for instance, castor oil. Compounds containing hydroxyl groups suchas alcohols or glycols, in particular cardol and/or cardanol, can beused as reactive thinners.

The amount of the other compounds containing hydroxyl groups depends onthe desired properties of the proppant coating and can suitably beselected by the person skilled in the art. Typical amounts of compoundscontaining hydroxyl groups are in the range of between about 10 wt % andabout 80 wt %, preferably from about 20 wt % to about 70 wt %, relativeto 100 wt % of the polyol component.

The process of the present invention is particularly useful when theproppants are coated with a condensation reaction product that has beenmade with an excess of isocyanate component with respect to the polyolor curing agent component. In step (a) therefore, 100 parts by weight ofthe polyol component is used with about 100 to about 600, preferablyabout 210 to about 530, more preferably about 220 to about 420, andstill more preferably about 230 to about 400 parts by weight of theisocyanate base value. Ratios of iso:polyol from about 10:90 to as lowas 100:0 are usable depending on the equipment, conditions andproduction rate provided that the coating and reaction are completedduring the coating process. The preferred range of iso:polyol isgenerally within the range from about 10:90 to about 90:10.

The isocyanate base value defines the amount of the isocyanate componentwhich is equivalent to 100 parts by weight of the polyol component. TheNCO-content (%) of the isocyanate component is defined herein accordingto DIN ISO 53185. To determine the OH— content (%) of the polyolcomponent, first the so-called OH-number is determined in mg KOH/gaccording to DIN ISO 53240 and this value is divided by 33, in order todetermine the OH— content. Thus, in step (a) an excess of NCO-groups inthe isocyanate component of between about 100 and about 500%, preferablyabout 110 to about 430%, more preferably about 120% to about 320%, andstill more preferably about 130% to about 300%, relative to theOH-groups in the polyol component is used.

Moreover, in step (a) one or more additives can be mixed with theproppant, the polyol component and the isocyanate component. Theseadditives are not particularly restricted and can be selected from theadditives known in the specific field of coated proppants. Provided thatone of these additives has hydroxyl, amine or amide functional groups,it should be considered as a different reactive group-containingcompound, as described above in connection with the polyol component. Ifone of the additives has isocyanate groups, it should be considered as adifferent isocyanate-group-containing compound. Additives with hydroxylgroups and isocyanate groups can be simultaneously considered asdifferent hydroxyl-group-containing compounds and as differentisocyanate-group-containing compounds.

Reactive Amines or Amides

The coating formulation of the present invention also optionallyincludes a reactive amine or reactive amide component, preferably anamine-terminated compound or an amide. The coating formulation can,however, be made effectively and with good properties in the absence orsubstantial absence of a reactive amine component apart from thereactive polyol and isocyanate components. The reactive amine componentcan enhance crosslink density within the coating and, depending oncomponent selection, can provide additional characteristics of benefitto the cured coating. Reactive amine components for use in the presentinvention include C1-C40 amine-terminated, amine-containing, or amidecompounds such as monoamines (e.g., butyl amine), amides (e.g., fattyacid amides, stearyl amides), diamines, triamines, amine-terminatedglycols such as the amine-terminated polyalkylene glycols soldcommercially under the trade name JEFFAMINE from Huntsman PerformanceProducts in The Woodlands, Tex. The use of amides can be particularlyuseful for enhancing flow and hydrophobic properties as well as theantimicrobial properties of the coatings.

Suitable diamines include primary, secondary and higher polyamines andamine-terminated compounds. Suitable compounds include, but are notlimited to, ethylene diamine; propylenediamine; butanediamine;hexamethylenediamine; 1,2-diaminopropane; 1,4-diaminobutane;1,3-diaminopentane; 1,6-diaminohexane; 2,5-diamino-2,5-dimethlhexane;2,2,4- and/or 2,4,4-trimethyl-1,6-diaminohexane; 1,11-diaminoundecane;1,12-diaminododecane; 1,3- and/or 1,4-cyclohexane diamine;1-amino-3,3,5-trimethyl-5-aminomethyl-cyclohexane; 2,4- and/or2,6-hexahydrotoluoylene diamine; 2,4′ and/or 4,4′-diaminodicyclohexylmethane and 3,3′-dialkyl-4,4′-diamino-dicyclohexyl methanes such as3,3′-dimethyl-4,4-diamino-dicyclohexyl methane and3,3′-diethyl-4,4′-diaminodicyclohexyl methane; aromatic polyamines suchas 2,4- and/or 2,6-diaminotoluene and 2,6-diaminotoluene and 2,4′ and/or4,4′-diaminodiphenyl methane; and polyoxyalkylene polyamines (alsoreferred to herein as amine terminated polyethers).

Mixtures of polyamines may also be employed in preparing asparticesters, which is a secondary amine derived from a primary polyamine anda dialkyl maleic or fumaric acid ester, for use in the invention.Representative examples of useful maleic acid esters include dimethylmaleate, diethyl maleate, dibutyl maleate, dioctyl maleate, mixturesthereof and homologs thereof.

Suitable triamines and higher multifunctional polyamines for use in thepresent coating include diethylene triamine, triethylenetetramine, andhigher homologs of this series.

JEFFAMINE diamines include the D, ED, and EDR series products. The Dsignifies a diamine, ED signifies a diamine with a predominatelypolyethylene glycol (PEG) backbone, and EDR designates a highlyreactive, PEG based diamine.

JEFFAMINE D series products are amine terminated polypropylene glycolswith the following representative structure:

JEFFAMINE ® x MW* D-230 ~2.5 230 D-400 ~6.1 430 D-2000 ~33 2,000 D-4000(XTJ-510) ~68 4,000

JEFFAMINE EDR-148 (XTJ-504) and JEFFAMINE EDR-176 (XTJ-590) amines aremuch more reactive than the other JEFFAMINE diamines and triamines. Theyare represented by the following structure:

JEFFAMINE ® y x + z MW* HK-511 2.0 ~1.2 220 ED-600 (XTJ-500) ~9.0 ~3.6600 ED-900 (XTJ-501) ~12.5 ~6.0 900 ED-2003 (XTJ-502) ~39 ~6.0 2,000

JEFFAMINE T series products are triamines prepared by reaction ofpropylene oxide (PO) with a triol intiator followed by amination of theterminal hydroxyl groups. They are exemplified by the followingstructure:

Moles PO JEFFAMINE ® R n (x + y + z) MW* T-403 C₂H₅ 1 5-6 440 T-3000(XTJ-509) H 0 50 3000 T-5000 H 0 85 5000

The SD Series and ST Series products consist of secondary amine versionsof the JEFFAMINE core products. The SD signifies a secondary diamine andST signifies a secondary trimine. The amine end-groups are reacted witha ketone (e.g. acetone) and reduced to create hindered secondary amineend groups represented by the following terminal structure:

One reactive hydrogen on each end group provides for more selectivereactivity and makes these secondary di- and triamines useful forintermediate synthesis and intrinsically slower reactivity compared withthe primary JEFFAMINE amines.

JEFFAMINE ® Base Product MW* SD-231 (XTJ-584) D-230 315 SD-401 (XTJ-585)D-400 515 SD-2001 (XTJ-576) D-2000 2050 ST-404 (XTJ-586) T-403 565

See also U.S. Pat. Nos. 6,093,496; 6,306,964; 5,721,315; 7,012,043; andPublication U.S. Patent Application No. 2007/0208156 the disclosures ofwhich are hereby incorporated by reference.

Additionally, the amine containing compound can be monofunctional asprimary amines and amides, each capable of incorporating desirableproperties into the coating, e.g., hydrophobic characteristics, betterflow properties and antimicrobial properties.

Additives

The proppant coating compositions of the invention may also includevarious additives. For example, the coatings of the invention may alsoinclude pigments, tints, dyes, and fillers in an amount to providevisible coloration in the coatings. Other materials include, but are notlimited to, reaction enhancers or catalysts, crosslinking agents,optical brighteners, propylene carbonates, coloring agents, fluorescentagents, whitening agents, UV absorbers, hindered amine lightstabilizers, defoaming agents, processing aids, mica, talc,nano-fillers, silane coupling agents, antislip agents, water affinity orrepulsion components, impact modifiers, water-activated catalysts,viscosifiers, flowaids, anticaking agents, wetting agents, tougheningagents such as one or more block copolymers, and components that act toremove at least some portion of the heavy metals and/or undesirablesolutes found in subterranean groundwater. See, copending U.S. patentapplication Ser. No. 13/224,726 filed on 1 Sep. 2011 entitled “DualFunction Proppants”, the disclosure of which is herein incorporated byreference. The additives are preferably present in an amount of about 15weight percent or less. In one embodiment, the additive is present in anon-zero amount of about 5 percent or less by weight of the coatingcomposition. Especially preferred are amorphous silica (e.g., silicaflour, fumed silica and silica dispersions) and silica alternatives(such as those used in sandblasting as an alternative to silica ororganofunctional silane like the DYNASYLAN fluids from Evonik DegussaCorporation in Chester, Pa.) that act as anticaking agents ordispersions that are applied to the outer surfaces of the coatedproppant solid to prevent the formation of agglomerates during packingand shipping. Applied amounts of the amorphous silica are generallywithin the range from about 0.001 wt % to about 1 wt % based on the dryproppant weight.

Other additives can include, for example, solvents, softeners,surface-active agents, molecular sieves for removing the reaction water,thinners and/or adhesion agents can be used. Silanes are a particularlypreferred type of adhesion agent that improves the affinity of thecoating resin for the surface of the proppant. Silanes can be mixed inas additives in step (a), but can also be converted chemically withreactive constituents of the polyol component or of the isocyanatecomponent. Functional silanes such as amino-silanes, epoxy-, aryl- orvinyl silanes are commercially available and, as described above, can beused as additives or can be converted with the reactive constituents ofthe polyol component or of the isocyanate component. In particular,amino-silanes and epoxy-silanes can be easily converted with theisocyanate component.

An optional additional additive is a contaminant removal component thatwill remove, sequester, chelate or otherwise clean at least onecontaminant, especially dissolved or otherwise ionic forms of heavymetals and naturally occurring radioactive materials (NORMS), fromsubterranean water or hydrocarbon deposits within a fractured stratumwhile also propping open cracks in said fractured stratum. Preferably,the contaminant removal component is associated with the proppant solidas a chemically distinct solid that is introduced together with theproppant solid as: (a) an insoluble solid secured to the outer or innersurface of the proppant solid with a coating formulation that binds thesolids together, (b) as a solid lodged within pores of the proppantsolid or (c) as a chemical compound or moiety that is mixed into orintegrated with a coating or the structure of the proppant solid. Seecopending U.S. patent application Ser. No. 13/224,726 filed on 2 Sep.2011 entitled “Dual Function Proppants” the disclosure of which isherein incorporated by reference. Additional added functionality canalso be in the form of frac fluid breakers, demulsifiers, andbactericides.

The added functionality of an auxiliary particle to the proppant mayalso be in the form of an ion exchange resin that is pretreated or whichitself constitutes a dissolvable solid for the slow release of corrosionor scale inhibitors. Such slow release materials could prove beneficialand advantageous to the overall operation and maintenance of the well.

Proppant Core Solids

The proppants can be virtually any small solid with an adequate crushstrength and lack of chemical reactivity. Suitable examples includesand, ceramic particles (for instance, aluminum oxide, silicon dioxide,titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide,manganese dioxide, iron oxide, calcium oxide, magnesium oxide, orbauxite), or also other granular materials.

Proppant sands are a preferred type of proppant for the presentinvention. Sand is mainly used in the hydraulic fracturing process ofnatural gas and oil wells to increase their productivity of valuablenatural resources. Proppant sand is monocrystalline with a high silicacontent of at least 80 wt %, and more typically greater than about 97 wt% silica.

American Petroleum Institute specifications place the followinglimitations on sieve distribution for proppants suitable for use in afracture:

-   -   At least 90% of material must fall between the two mesh sizes,    -   No more than 10% of the material may be coarser than the largest        mesh size,    -   No more than 0.1% of the material may be coarser than the next        largest mesh size [e.g. for 20/40, up to 10% of the proppant may        be between 16 and 20 mesh, but no more than 0.1% can exceed 16        mesh], and    -   No more than 1% of material is permitted to fall onto the pan.

According to bulk density, proppant is divided into: low-density, mediumdensity, high-density. According to the anti-crushing strength, proppantis divided into 52 Mpa, 69 Mpa, 86 Mpa and 103 Mpa four series.Specifications of proppant sand are generally: 12-18 mesh, 12-20 mesh,16-20 mesh, 16-30 mesh, 20-40 mesh between 30-50 mesh, 40-60 mesh, 40-70mesh and smaller. The proppants to be coated preferably have an averageparticle size within the range from about 50 μm and about 3000 μm, andmore preferably within the range from about 100 μm to about 2000 μm.

Coating Method

The method for the production of coated proppants according to thepresent invention can be implemented without the use of solvents.Accordingly, the mixture obtained in step (a) in one embodiment of themethod is solvent-free, or is essentially solvent-free. The mixture isessentially solvent-free, if it contains less than 20 wt %, preferablyless than 10 wt %, more preferably less than 5 wt %, and still morepreferably less than 3 wt %, and most preferably less than 1 wt % ofsolvent, relative to the total mass of components of the mixture.

Preferably, the method is implemented without the use of organicsolvents. In this case, the mixture obtained in step (a) is free oforganic solvents, or is essentially free of organic solvents. Themixture is essentially free of organic solvents, if it contains lessthan 20 wt %, preferably less than 10 wt %, more preferably less than 5wt %, and still more preferably less than 3 wt %, and most preferablyless than 1 wt % of solvent, relative to the total mass of components ofthe mixture.

In step (a) the proppant is preferably heated to an elevated temperatureand then contacted with the coating components. Preferably, the proppantis heated to a temperature within the range of about 50° C. to about150° C. to accelerate crosslinking reactions in the applied coating.

The temperature of the coating process is not particularly restrictedoutside of practical concerns for safety and component integrity. Thepreferred conditions for the coating/curing step of the presentinvention are generally at conditions within the range of about 50° toabout 175° C., more preferably at a temperature within the range fromabout 75° C. to about 150° C., and most preferably at a temperaturewithin the range from about 80° C. to about 135° C. This temperatureavoids a number of emissions issues, reduces the amount of energyconsumed in the coating process and also reduces the cooling time forthe coated proppants for further handling and packaging.

The mixer used for the coating process is not particularly restrictedand can be selected from among the mixers known in the specific field.For example, a pug mill mixer, agitation mixer, drum mixer, plate-typemixer, tubular mixer, trough mixer or conical mixer can be used. Theeasiest way is mixing in a rotating drum. As continuous mixer, a wormgear can, for example, be used.

Mixing can be carried out on a continuous or discontinuous basis. Insuitable mixers it is possible, for example, to add adhesion agents,isocyanate, amine and optional ingredients continuously to the heatedproppants. For example, isocyanate components, amine reactant andoptional additives can be mixed with the proppant solids in a continuousmixer (such as a worm gear) in one or more steps to make one or morelayers of cured coating.

Preferably, the proppant, isocyanate component, curing agent, aminereactant, polyol, and optional additives are mixed homogeneously. Thus,the isocyanate component and amine reactant are distributed uniformly onthe surface of the proppants. The coating ingredients are preferablykept in motion throughout the entire mixing process.

It is also possible to arrange several mixers in parallel, series, orserially in several runs in one mixer.

Importantly, the time, temperature, chemistry and reaction rate of thecoating/curing process can be combined in proportions that will affectperformance characteristics of the resulting cured coating. Preferably,an isocyanate-containing component is used in an amount within the rangefrom about 100 wt % to about 400 wt % based on a reactivepolyol-containing component in the curable coating mixture. Lowerproportions of excess isocyanate can be used to move the curing processtowards substantially complete reaction of all of the —NCO groups withinthe applied proppant coating by the time the product is discharged as afree-flowing solid. The lower amount of isocyanate-containing componenttend to add more thermoplastic properties to the coating for betterperformance in low temperature applications. Preferably, the proppantcoating is cured to an amount less than about 10 wt % of reactive —NCOgroups based on the originally applied weight of the proppant coating.The most preferred low temperature proppants according to the inventioncontain have a weight ratio of isocyanate-functional component that iswithin the range from about 100-175 wt % of the polyol-functionalcomponent with a low coating loss under simulated downhole testingconditions.

Having more unreacted —NCO groups can be useful to develop morethermoset characteristics in the coating thereby making the proppantbetter suited for high temperature applications. In such a case, ahigher amount of isocyanate is used. The preferred high temperatureproduct contains about 200-400% by weight of isocyanate-functionalcomponent to polyol-functional component and exhibits a coating loss ofless than about 2% in simulated downhole testing conditions.

The coating is preferably performed at the same time as the curing ofthe coating on the proppant. In the present invention, the coatedproppant becomes free-flowing at a time of less than 5 minutes,preferably within the range of 1-4 minutes, more preferably within therange of 1-3 minutes, and most preferably within the range of 1-2.5minutes to form a coated, substantially cured, free-flowing, coatedproppant. This short cycle time combines with the relatively moderatecoating temperatures to form a coating/curing process that provideslower energy costs, smaller equipment, reduced emissions from theprocess and the associated scrubbing equipment, and overall increasedproduction for the coating facility.

The coating material may be applied in more than one layer. In thiscase, the coating process is repeated as necessary (e.g. 1-5 times, 2-4times or 2-3 times) to obtain the desired coating thickness. A typicalsize range for coated proppant is typically within the range of about 16to about 100 mesh.

The amount of coating resin, that is, of the polyurethane resin appliedto a proppant, is preferably between about 0.5 and about 10 wt %, morepreferably between about 1% and about 5 wt %, resin relative to the massof the proppant as 100 wt %. With the method according to the presentinvention proppants can be coated at temperatures between about 10° C.and about 150° C. and preferably in a solvent-free manner. The coatingprocess requires a comparatively little equipment and if necessary canalso be carried out near the sand or ceramic substrate source, near thegeographically location of the producing field or at/near the wellitself.

The coated proppants can additionally be treated with surface-activeagents, anticaking agents, or auxiliaries, such as talcum powder orstearate or other processing aids such as fine amorphous silica toimprove pourability, wettability (even to the extent that a waterwetting surfactant can be eliminated), dispersability, reduced staticcharge, dusting tendencies and storage properties of the coated product.

If desired, the coated proppants can be baked or heated for a period oftime sufficient to further enhance the ultimate performance of thecoated particles and further react the available isocyanate, hydroxyland reactive amine groups that might remain in the coated proppant. Sucha post-coating cure may occur even if additional contact time with acatalyst is used after a first coating layer or between layers.Typically, the post-coating cure step is performed like a baking step ata temperature within the range from about 100°-200° C. for a time ofabout 1 minute to 4 hours, preferably the temperature is about 125°-200°C. for-1-30 minutes.

Even more preferably, the coated proppant is cured for a time and underconditions sufficient to produce a coated proppant that exhibits a lossof coating of less than 25 wt %, preferably less than 15 wt %, and evenmore preferably less than 5 wt % when tested according to simulateddownhole conditions under ISO 13503-5:2006(E). Even more preferably, thecoated proppant exhibits the low dust and handling characteristics of apre-cured proppant (see API RP 60) but also exhibits a crush test resultat 10,000 psi of less than 2%, more preferably less than 1.5%, andespecially less than 1%. The coated proppants of the inventionpreferably also have an unconfined compressive strength of greater than100 psi and more preferably more than 500 psi with a fractureconductivity at a given closure stress that is substantially equal to,or greater than, the conductivity of a phenolic coating used in the sameproduct application range.

Using the Coated Proppants

The invention also includes the use of the coated proppants inconjunction with a fracturing liquid to increase the production ofpetroleum or natural gas. Techniques for fracturing an unconsolidatedformation that include injection of consolidating fluids are also wellknown in the art. See U.S. Pat. No. 6,732,800 the disclosure of which isherein incorporated by reference. Generally speaking, a fluid isinjected through the wellbore into the formation at a pressure less thanthe fracturing pressure of the formation. The volume of consolidatingfluid to be injected into the formation is a function of the formationpore volume to be treated and the ability of the consolidating fluid topenetrate the formation and can be readily determined by one of ordinaryskill in the art. As a guideline, the formation volume to be treatedrelates to the height of the desired treated zone and the desired depthof penetration, and the depth of penetration is preferably at leastabout 30 cm radially into the formation. Please note that since theconsolidation fluid is injected through the perforations, the treatedzone actually stems from the aligned perforations.

Before consolidating the formation, according to a preferred embodiment,an acid treatment is performed by injection of an acidic fluid. As it iswell known in the art, this acidic treatment typically includes severalstages such as an acid preflush, one or more stages of acid injectionand an overflush.

After the perforation and the consolidation, the final step is thefracturing step. Although a resin treatment alone may have beensufficient in preventing early sand production the resin reduces thepermeability of the formation around the wellbore. The primary purposeof the fracture treatment is to connect the wellbore to the formationand in doing so by pass any damage and act as a filter allowing theproduction of hydrocarbons while holding back formation material. Thehigh surface area associated with a fracture makes it a very effectivefilter, for example, a 13.7 m fracture length with 25 cm height has asurface area of 368 m², compared to the open hole flow area for a gravelpack of 3.2 m² with the same zone height.

Techniques for hydraulically fracturing a subterranean formation will beknown to persons of ordinary skill in the art, and will involve pumpingthe fracturing fluid into the borehole and out into the surroundingformation. The fluid pressure is above the minimum in situ rock stress,thus creating or extending fractures in the formation. In order tomaintain the fractures formed in the formation after the release of thefluid pressure, the fracturing fluid carries a proppant whose purpose isto prevent the fracturing from closing after pumping has been completed.

The fracturing liquid is not particularly restricted and can be selectedfrom among the frac liquids known in the specific field. Suitablefracturing liquids are described, for example, in W C Lyons, G J Plisga,Standard Handbook Of Petroleum And Natural Gas Engineering, GulfProfessional Publishing (2005). The fracturing liquid can be, forexample, water gelled with polymers, an oil-in-water emulsion gelledwith polymers, or a water-in-oil emulsion gelled with polymers. In onepreferred embodiment, the fracturing liquid comprises the followingconstituents in the indicated proportions: 10001 water, 20 kg potassiumchloride, 0.120 kg sodium acetate, 3.6 kg guar gum (water-solublepolymer), sodium hydroxide (as needed) to adjust a pH-value from 9 to11, 0.120 kg sodium thiosulfate, 0.180 kg ammonium persulfate andoptionally a crosslinker such as sodium borate or a combination ofsodium borate and boric acid to enhance viscosity.

In addition, the invention relates to a method for the production ofpetroleum or natural gas which comprises the injection of the coatedproppant into the fractured stratum with the fracturing liquid, i.e.,the injection of a fracturing liquid which contains the coated proppant,into a petroleum- or natural gas-bearing rock layer, and/or itsintroduction into a fracture in the rock layer bearing petroleum ornatural gas. The method is not particularly restricted and can beimplemented in the manner known in the specific field.

Suitable proppants include, but are not limited to, sand, bauxite, glassbeads, and ceramic beads and resin-coated versions of each. The proppantwill typically exhibit a size within the range from about 8 to about 100U.S. Standard Mesh in size. Mixtures of suitable proppants can be used.The concentration of proppant in the fracturing fluid can be anyconcentration known in the art, and will typically be in the range ofabout 0.5 to about 20 pounds of proppant added per gallon of cleanfluid.

The fracturing fluid can contain an added proppant-retention agent, e.g.a fibrous material, a curable resin coated on the proppant, platelets,deformable particles, or a sticky proppant coating to trap proppantparticles in the fracture and prevent their production through thewellbore. Fibers, in concentration that preferably ranges from about0.1% to about 5.0% by weight of proppant, for example selected fromnatural organic fibers, synthetic organic fibers, glass fibers, carbonfibers, ceramic fibers, inorganic fibers, metal fibers and mixturesthereof, in combination with curable resin-coated proppants areparticularly preferred. The proppant-retention agent is intended to keepproppant solids in the fracture, and the proppant and proppant-retentionagent keep formation particles from being produced.

EXAMPLES

Conductivity testing was performed at simulated downhole conditionsusing the method and procedures found in ISO 13503-5:2006. In suchtests, a closure stress is applied across a test unit for 50 hours toallow the proppant sample bed to reach a semi-steady state condition.Initially the pack is allowed something around 16 hours to stabilize at1000 psi closure stress and the test temperature before elevating theclosure stress on the proppant. As the fluid is forced through theproppant bed, the pack width, differential pressure, pressure drop,temperature and flow rates are measured at each stress. Proppant packpermeability and conductivity are then calculated.

Multiple flow rates are used to verify the performance of thetransducers, and to determine Darcy flow regime at each stress; anaverage of the data at these flow rates is reported. The test fluid is 2wt % potassium chloride substitute solution filtered to 3 μm absolute.The initial conductivity, permeability and width is measured andcompared to the final conductivity, permeability and width after eachstress period. Stress is applied and maintained using an Isco 260D.Stress is applied at 100 psi/minute.

Width of the proppant pack is determined by assembling the conductivitycell with the Ohio sandstone wafers and shims without the sampleproppants. The distance between the width bars that are attached to eachend of the conductivity cells are measured at each of the four cornersand recorded. The cells are then assembled with the proppant samples.The measurements are made again at the beginning and ending of eachstress period. Width is determined by subtracting the average of thezero from the average of each of the stress width values. Conductivityis calculated using Darcy's equation.

kW _(f)=26.78 μQ/(ΔP)  Conductivity;

k=321.4 μQ/[(Δ P)W _(f)]  Permeability;

wherein:

k is the proppant pack permeability, expressed in Darcy's;

kW_(f) is the proppant pack conductivity, expressed in millidarcy-feet

μ is the viscosity of the test liquid at test temperature, expressed incentipoises;

Q is the flow rate, expressed in cubic centimeters per minute;

ΔP is the differential pressure, expressed in psi;

W_(f) is proppant pack width, expressed in inches.

Sieve analysis is performed using the procedure found in ISO 13503-2“Measurements of proppants used in hydraulic fracturing and gravel packoperations” Standard US mesh screens are used to separate the sample bysize. Not more than 0.1% should be greater than the first specifiedsieve and not more than 1% should be retained in the pan. There shouldbe at least 90% retained in the specified screens.

To determine the magnitude of coating loss during the conductivity test,samples of the proppant pack are taken, dried in an oven and weighed.They are then subjected to a temperature of 960° C. for 2.5 hours. Atthe end of this period the samples are cooled and weighed again. Thedifference between the sample weight after drying but before beingsubjected to the furnace compared to the sample weight after the time inthe furnace, equates to the coating weight. Comparing this number to thesame test performed on a sample of the coated material before beingsubjected to the conductivity test, will equate to the coating weightlost due to the long term exposure to the conditions of the conductivitytests.

The procedure used in an autoclave test would be as follows:

The autoclave test utilizes what amounts to a pressure cooker to subjectthe coated sands to a hot wet environment that is above the boilingtemperature of water. Approximately 20 g of sample is placed in a jaralong with 150 ml of distilled water. The lids are placed on sample jarsbut not tightened. The samples are placed in the autoclave and thechamber is sealed. Heat is applied until the autoclave temperaturereaches 250-265° F. (121°-129° C.). The samples are maintained underthese conditions for the ten day period. At the end of the test periodthe autoclave is cooled down, opened and the sample jars removed. Eachsample is washed with distilled water and then placed in an oven to dry.The dried samples are then put through a standard test for determinationof coating loss. This result is compared to the results of a coatingtest that was run on the original sample. The difference in coatingweight before and after the autoclave test, quantifies the amount ofcoating that was dissolved by the exposure to a hot water environment.

Example 1

Ten pounds of Minnesota 40/70 fracturing sand is heated to 200° F. in alaboratory mixer at which point the following components are added inthe sequence and timing as given below in Tables 1 and 2. The weightratio of the poly-MDI to phenolic polyol in this example is 75/25.

TABLE 1 WEIGHT (grams) COMPONENT 4540 Minnesota sand 4.5 silane couplingagent 2.3 50% red iron oxide in castor oil 6.91,3,5-tris(3-(dimethylamino)propyl)-hexahydro-s- triazine (JEFFCAT TR90)34.2 Phenolic Polyol comprising 48% phenolic resin, 28% cashew nut oil,24% castor oil 102.2 Poly-MDI (32% NCO content) 2.5 Wetting agent

TABLE 2 TIME (minutes:seconds) ADDITION/COMMENT 0:00 Sand is at 200° F.0:00 4.5 gms silane coupling agent is added over 30 secs 0:00 JEFFCATTR90 is added over 20 secs 0:00 Blend of red iron oxide and PhenolicPolyol is added over 60 seconds 0:10 Poly-MDI is added over 60 seconds2:00 Product is free flowing 3:30 Wetting agent is added over 5 seconds4:00 Product is discharged at 180° F.

In this and the other examples presented herein, it was noticed that theJEFFCAT TR90 catalyst increased the reaction rate sufficiently that theamperage on the associated mixer was not exceeded as the coatingreactants were metered into the proppant solids in the moving mixer.This suggests that the coating became cured at a rate that wasconsistent with the feed rate so that the liquid viscosity did notincrease the electrical load on the mixer. This same method ofcontrolled, metered addition would also apply for other formulations andchemistries under the present invention in order to keep the contentsreacting at a rate that does not tax the load on the mixing equipment.

The resin coated sand from the example above tested at 2.75% coatinglevel from the mixer. When subjected to a three day 250° F. autoclavetest, the coating level was measured again at 2.34% reflecting the goodresistance to hot water removal of the coating.

Example 2

Ten pounds of Genoa 40/70 fracturing sand is heated to 204° F. in alaboratory mixer at which point the following components are added inthe sequence and timing as given below in Tables 3 and 4. The weightratio of the poly MDI to phenolic polyol in this example is 75/25.

TABLE 3 WEIGHT (grams) COMPONENT 4540 Genoa sand 4.5 A1100 silanecoupling agent 2.3 50% red iron oxide in castor oil 6.9 JEFFCAT TR9018.2 Phenolic Polyol comprised of 48% phenolic resin, 28% cashew nutoil, 24% castor oil 54.5 Poly-MDI (32% NCO content) 2.3 Wetting agent

TABLE 4 TIME (minutes:seconds) ADDITION/COMMENT 0:00 Sand is at 204° F.0:00 4.5 gms A1100 is added over 10 secs 0:00 JEFFCAT TR90 is added over10 secs 0:00 blend of red iron oxide and Phenolic Polyol is added over30 seconds 0:10 polyMDI is added over 30 seconds 2:00 Product isfree-flowing 3:30 Wetting agent is added over 5 seconds 4:00 Product isdischarged at 182° F.

The resin coated sand from the example above tested as having 1.48%coating from the mixer. When subjected to a three day 250° F. autoclavetest, the coating level was measured again at 1.43% reflecting the goodresistance to hot water removal of the coating.

Example 3

Ten pounds of Minnesota fracturing sand is heated to 200° F. in alaboratory mixer at which point the following components are added inthe sequence and timing as given below in Tables 5 and 6. The weightratio of the poly MDI to phenolic polyol in this example is 92/8.

TABLE 5 WEIGHT (grams) COMPONENTS: 4540 Minnesota sand 4.5 A1100 silanecoupling agent 2.3 50% red iron oxide in castor oil 6.9 JEFFCAT TR 90 12Phenolic Polyol comprised of 48% phenolic resin, 28% cashew nut oil, 24%castor oil 135 Poly-MDI (32% NCO content) 2.5 Wetting agent

TABLE 6 TIME (minutes:seconds) ADDITION/COMMENT 0:00 Sand is at 202° F.0:00 A1100 is added over 20 secs 0:00 JEFFCAT TR90 is added over 20 secs0:00 Blend of red iron oxide and Phenolic Polyol is added over 60seconds 0:10 Poly-MDI is added over 60 seconds 2:00 Product is freeflowing 3:30 Wetting agent is added over 5 seconds 4:00 Product isdischarged at 170° F.

The resin coated sand from example 3 tested at 2.80% coating level fromthe mixer. When subjected to a three day 250° F. autoclave test, thecoating level was measured again at 2.56% reflecting the good resistanceto hot water removal of the coating.

Example 4

One kg of 40/70 Minnesota fracturing sand is heated to 210° F. in alaboratory mixer at which point the following components are added inthe sequence and timing as given below in Tables 7 and 8. The weightratio of the poly-MDI to the aminated polyalkyleneglycol (JEFFAMINED230) is 63/37.

TABLE 7 WEIGHT (grams) COMPONENTS 1000 Minnesota sand 1 A1100 silanecoupling agent 20 Poly-MDI (32% NCO content) 12 JEFFAMINE D230

TABLE 8 TIME (minutes:seconds) ADDITION/COMMENT 0:00 Sand is at 210° F.0:00 A1100 is added over 10 secs 0:10 Poly-MDI is added over 30 secs0:50 JEFFAMINE D230 is added over 10 secs 2:00 Product is free flowing4:00 Product is discharged at 140° F.

The resin coated sand from the example above tested at 2.90% coatinglevel from the mixer. When subjected to a three day 250° F. autoclavetest, the coating level was measured again at 2.83% reflecting the goodresistance to hot water removal of the coating.

Example 5

One kg of 40/70 Minnesota fracturing sand is heated to 210° F. in alaboratory mixer at which point the following components are added inthe sequence and timing as given below in Tables 9 and 10. The weightratio of the poly-MDI to the aminated polyalkyleneglycol (JEFFAMINE D230from Huntsman Corporation) is 63/37.

TABLE 9 WEIGHT (grams) COMPONENTS 1000 Minnesota sand 1 A1100 silanecoupling agent 20 Poly-MDI (32% NCO content) 12 Aminatedpolyalkyleneglycol 0.6 Triethylenediamine

TABLE 10 TIME (minutes:seconds) ADDITION/COMMENT 0:00 Sand is at 210° F.0:00 A1100 is added over 10 secs 0:10 Poly-MDI is added over 30 secs0:50 Preblended Aminated polyalkyleneglycol and Triethylenediamine areadded over 10 s 2:00 Product is free flowing 4:00 Product is dischargedat 145° F.

The resin coated sand from the example above tested at 2.84% coatinglevel from the mixer. When subjected to a three day 250° F. autoclavetest, the coating level was measured again at 2.63% reflecting the goodresistance to hot water removal of the coating.

Example 6

In this example, a series of test results were performed to demonstratethe properties of proppant coatings that include completely reacted(pre-cured) and partially cured phenolic coatings as compared to thecoating of the present invention (“new technology coating”). The graphin FIG. 1 illustrates the TMA results for (a) Pre-cured phenolic coatedsand, (b) New Technology coated sand using the formulation of Example 1,(c) a partially cured, phenolic-coated sand (also identified as PhenolicA) and (d) a somewhat more curable, phenolic-coated sand (alsoidentified as Phenolic B)

The ThermoMechanical Analyzer (TMA) as supplied by TA Instruments is adevice that accurately imposes a small force (i.e., a load) onto asample which is then subjected to a desired temperature ramp over adefined time. During this increasing temperature period, the force isheld constant. The probe which imposes the force is connected to asophisticated micrometer that is capable of measuring fractions of amicron change in the position of the probe. Any change in the positionof the probe can be interpreted to reflect an expansion or contractionof the sample that is brought about by the temperature change(s). Inmany applications, the sample merely expands as it is being heated (forinstance raw sand) thereby creating a database that refers to thecoefficient of thermal expansion. The TMA has the ability to run thesamples in a variety of environments.

In general, a pre-cured, phenolic-coated sand will be characterized by aplot that is essential flat (parallel to the X axis) or has a positiveslope. This response is indicative of a coating that is essentiallyreacted in which there is little to no remaining reactivity that remainsin the coating.

If, however, a more curable or partially cured phenolic coating istested, the TMA plot will exhibit a negative slope as early as 80° C. to100° C., but more often after about 125° C. to about 175° C. This typeof plot is characteristic of a coating that has retained a level ofreactivity even after completing the manufacturing process. The morenegative the slope and the lower the temperature in which the slopeturns negative, the more reactivity that has been left in the coating.

As shown in FIG. 1, the top curve is labeled “Pre-cured” and isindicative of the response of a phenolic coating that is no longerreactive. The two lower curves are labeled “Partially Cured Phenolic A”and “More Curable Phenolic B.” These curves represent the TMA resultsfrom two levels of partially cured coatings. The second curve labeled“New Technology” shows a response that is similar to the pre-curedcoating curve but actually shows properties that fall between apre-cured coating and the less reactive partially cured coating. Theshape of the New Technology curve indicates that the New Technologycoating would exhibit some properties that are similar to a pre-curedcoating and others that may be similar to the partially cured coatings.

The plot of “Crush Results” in FIG. 2 illustrates the comparablestrength of sand coated with pre-cured phenolic coating, two partiallycured coatings (labeled A and B) and the New technology coating.Historically, the pre-cured phenolic coated sand would show a lowercrush percentage (in the ISO test procedure) than partially cured coatedsand. These crush test results follow this trend with the pre-curedcoating sand having a crush of 2.05% and the two partially coatings(Phenolic coatings A and B) having crushes of 3.93% and 4.95%respectively. It is important to notice that the coating having the moreremaining reactivity (Phenolic B) has the highest crush value. The NewTechnology coating actually tested out having the lowest crush value(0.69%). So in the crush evaluation The New Technology coating performedlike a superior, pre-cured coated sand.

The plot entitled “Unconfined Compressive Strength” in FIG. 3 representsa strength measurement of the particle to particle bonds of a coatedproppant sand. Historically, a pre-cured coated phenolic sand possesslittle if any ability to form particle to particle bonds of anymeasureable strength. In this test the coating labeled Phenolic Aexhibited a bond strength UCS of 449 psi. The coating labeled Phenolic Bhad a UCS of 155 psi. Since the TMA indicated that Phenolic B was a morereactive coating than Phenolic A, one would expect that the UCS resultsshould be reversed. That would be true if the coated sands had the sameresin level (LOT). However the plot entitled “Coated Sand Loss OnIgnition” in FIG. 5, shows that Phenolic A actually has a 3.97% phenoliccoating while phenolic coating B has a 2.84% resin coating. This couldbe one explanation for the unexpected UCS results.

In FIG. 3, the pre-cured phenolic coating sand showed only a weakbonding capability with a measurement of 7 psi. This level of bondingwould indicate that the pre-cured phenolic coating is not capable offorming particle-to-particle bonds that would consolidate the proppantor be effective in controlling proppant flowback.

The New Technology coating exhibited the TMA appearance of a pre-curedcoated sand in FIG. 1, it yielded the highest bond strength (UCS=576psi) of any sample tested. See FIG. 3. These dual results are new andunexpected for a coated proppant.

The plot entitled “Fracture Conductivity @ 4000 psi” in FIG. 4 reveals adata point from a long term conductivity test. Presented on the plot arethe conductivity numbers for the two partially cured phenolic coatedsands and a sand coated with the New Technology. Historically, theconductivity test results for a partially cured phenolic coating willmeet or exceed that of a pre-cured coating. The plot shows that the NewTechnology coating has a conductivity similar to the partially curedcoating of Phenolic A and superior to partially cured coating ofPhenolic B. This is in spite of the fact the Phenolic A has asignificantly higher coating level than the New technology and PhenolicB is marginally higher than the New Technology coating (see FIG. 5).

In summary, The New Technology coated sand exhibits the thermalproperties of a pre-cured phenolic coated sand and crush resistancesuperior to a pre-cured phenolic coating. It also shows a bondingcapability superior to the partially cured phenolic coated sand and acomparable if not superior fracture conductivity (when measured at 4,000psi in a long term conductivity test). This would seem to indicate thatthe new technology contains traits and performance properties from bothtype coating and would best be described as a “hybrid” coatingtechnology.

Example 7

A proppant coating formulation for Example 7 was prepared with anIso:Polyol ratio of 0.65 equivalent weight at a process temperature of198° F. (92° C.) and made from the curable coating ingredients shown inTable 11:

TABLE 11 INGREDIENT WEIGHT (LBS.) Sand 1000.00 Dynasylan AMEO (Silane)*1.00 Red 2B in Castor Oil 1.00 Dow 801X Polyol 19.34 Dow ISO(XUS17557.00) 20.11 Dabco T-12 (DBTDL) 0.17 SiO2•OH  2-12 Chemicals43.62-53.62 *Dynasylan ® AMEO from Evonik Degussa Corporation inChester, PA is a bifunctional silane possessing a reactive primary aminogroup and hydrolyzable ethoxysilyl groups. The dual nature of itsreactivity is represented by its manufacturer to allow Dynasylan ® AMEOto bind chemically to both inorganic materials (e.g. glass, metals,fillers) and organic polymers (e.g. thermosets, thermoplastics,elastomers) thus functioning as an adhesion promoter, crosslinker,and/or surface modifier.

Table 12 shows the timing and duration for the order of addition of thecomponents making up the curable coating mixture of the invention.

TABLE 12 COMPONENT START TIME (s) DURATION (s) END TIME (s) Silane 0 3 3Color 5 10 15 Polyol & catalyst 20 60 80 Isocyanate 30 60 90 Additive120 20 140 Discharge 180

What results from the above coating and curing process is asubstantially cured and coated proppant having handling characteristicslike a pre-cured, resin-coated proppant but with the ability to forminterparticle bonds under downhole conditions like a curable,resin-coated proppant. The resulting product is then further contactedwith a finely divided anticaking agent, like an amorphous silica orsilica substitute in dry form or as a dispersion.

The preferred anticaking agents are either a dry form of very smallamorphous silica or a dispersion of nanometer-sized fumed silica. Thefollowing Table 13 summarizes the differences between the additives:

TABLE 13 ANTICAKING % SURFACE PARTICLE STARTING LBS/1000 AGENT SOLIDSAREA SIZE CONC. WT. % LBS Colloidal silica 30 Medium- 50-100 nm 0.30010.00 High 1^(st) Dispersion (aq) 15 Low <20 nm 0.075 5.00 of fumedsilica 2nd Dispersion (aq) 20 Medium <20 nm 0.075 3.75 of fumed silica3^(rd) Dispersion (aq) 30 Low <20 nm 0.075 2.50 of fumed silica

Tests of unconfined compressive strength with a conventional proppanttester at 125° F. (52° C.), 24 hour shut-in, 1000 psi with a 2 wt % KClsolution and without use of a bond activator plasticizer show that a16/30 size of coated proppant sand according to the invention exhibitsan unconfined compressive strength of 100 psi, and a 20/40 size blend ofcoated sand exhibits an unconfined compressive strength of 92 psi.Comparative tests against similarly sized proppants that use apartially-cured phenolic resin coating and 1.5 wt % of a bond activatorplasticizer show no unconfined compressive strength under the sameconditions. In other words, the coated proppant of the invention forms ashaped sample exhibiting interparticle bonding of 92-100 UCS while thephenolic resin proppant remains loose particulates that show nointerparticle bond strength even though an activator was added topromote such bonds.

Some time is necessary before adequate interparticle bonds are developedwhen using the present invention. The bonds do not form instantaneouslyin low temperature wells at 100-125° F. (38°-52° C.). Generally, atleast about 5 hours is desirable with at least 12 hours is useful formost low temperature wells with proppants according to the presentinvention. This is referred to in the industry as the “shut-in” time inwhich the proppant is subjected to downhole conditions that reflecthealing and crack closure of the fractured field strata which exertcompressive stresses on the coated proppants within the fracture cracks.

FIGS. 6 and 7 show the results of comparative conductivity testsreflected in Table 14 below that compare the proppants of Example 7against two proppants with partially cured phenolic coatings. FIGS. 6and 7 show that the 16/30 proppant of the invention exhibits a 79%greater conductivity at 2000 psi and a 32% higher conductivity at 4000psi than the prior art phenolic proppant coating. FIG. 7 shows similarresults with the 20/40 proppant with a 29% higher conductivity at 2000psi and 13% greater at 4000 psi.

TABLE 14 Conductivity (md-ft) CLOSURE STRESS (psi) SAMPLE 1K 2K 4K 6K20/40 Invention (Low Temp Cured) 4703 3574 2117 1192 20/40 Competitor A3615 2773 1877 1317 16/30 Competitor B 6014 4826 2970 1735 16/30Invention (Low Temp Cured) 13563 8640 3945 1701

Hot water leaching tests show that the proppant coatings of the presentinvention show that the coating is highly resistant to leaching oncomponents and unreacted materials. Indeed, the test water after thetest was classifiable as safe to the limits of tap water for drinking.This contrasts with many phenolic coatings that can leach phenols andformaldehyde after prolonged exposure to hot water.

Example 8

High temperature wells present other performance issues for coatedproppants and the formation of a consolidated pack yet there is acontinued need for proppant coatings that can produce interparticlebonds despite extended exposure to an elevated temperature of at least200° F. (93° C.) for a period of time, e.g., at least about two hours orlonger, without interparticle contact due to closure stress while alsoreducing the generation of loose fine, resisting cyclic stress and beingcompatible with frac fluids, breakers and environmental concerns. Thecoating of this example is specifically directed to a coating that iswell suited to higher temperature wells.

A 30/50 sand coating according to the invention was prepared having theingredients in the curable mixture as shown in Table 15 in amountssufficient to form a 1.75 wt % coating on the underlying sand coresolid. The sand was pre-heated to 210° F. (99° C.).

TABLE 15 INGREDIENTS AMOUNT (GMS) Sand 2000.00 Polyol 7.90 DynasylanAMEO 2.00 Distilled Water 0.00 ISO 23.70 Chembetain CAS Surfactant 3.00Dabco TMR 0.21 Dabco T-12 0.10 Black colorant in Castor Oil 2.00 NanoArcAL-2125 0.00 Green colorant in Castor Oil 0.00 Chemical 38.92 Total2038.92

The ingredients of the curable mixture were added at the times and forthe durations shown in Table 16.

TABLE 16 INGREDIENT START (S) DURATION (S) END (S) Silane 0 5 5Colorants in Oil 5 5 10 Catalyst + Polyol 20 60 80 ISO 30 60 90 CAS 95 5100 Discharge 180

The coated sand of Example 8 and two prior art proppants with partiallycured phenolic coatings were then subjected to an unconfined compressivestrength test using the equipment and materials described in Example 7but operated at 250° F. (121° C.). In one set of conditions, theproppants were subjected to a three hour preheating that would becharacteristic of the exposure times and temperatures found in a hightemperature well. A comparison set of tests was performed without thepreheating to gauge the ability of the proppant to resist the effects ofextended exposure to heat before closure stress was applied. The resultsare shown in FIG. 8.

Inspection of FIG. 8 will show that both prior art coated proppants(Competitive Product 1 and Competitive Product 2) exhibit good UCS whenthere was no preheating and substantially diminished UCS when preheatingwas experienced. In contract, the coated proppant of the inventionexperienced consistent performance that was the same as, or better than,the prior art proppant products.

Once those skilled in the art are taught the invention, many variationsand modifications are possible without departing from the inventiveconcepts disclosed herein. The invention, therefore, is not to berestricted except in the spirit of the appended claims

1. A method for the production of coated proppant, comprising coating aproppant solid with a curable coating formulation and then allowing thecurable coating formulation to react under conditions sufficient tosubstantially cure said proppant coating, wherein said curable coatingformulation comprises a substantially homogeneous coating mixture thatcomprises (i) an isocyanate-functional component having at least 2isocyanate groups, and (ii) a curing agent comprising a monofunctionalalcohol, monofunctional amine or monofunctional amide, wherein saidcoating occurs simultaneously with said curing at a temperature and in aperiod of time of less than about four minutes to form a free-flowing,substantially cured, coated proppant.
 2. The method according to claim1, wherein said proppant solid comprises ceramic particles or sand.
 3. Amethod according to claim 1, wherein said proppant solid exhibits anaverage particle size within a range from about 50 μm to about 3000 μm.4. A method according to claim 1 wherein said curable coatingformulation further comprises an aliphatic polyol.
 5. A method accordingto claim 1, wherein said coating step is carried out at a temperaturewithin the range from about 50° C. to about 175° C.
 6. A methodaccording to claim 1 wherein said coating occurs simultaneously withsaid curing at a temperature within the range of 75° to about 150° C. 7.A method according to claim 1 wherein said curing agent furthercomprises an amine-based curing agent.
 8. A method according to claim 1,wherein the coated and cured proppant is contacted with an anticakingagent.
 9. A method according to claim 8 wherein said anticaking agentcomprises amorphous silica.
 10. A substantially fully cured, coated,proppant solid comprising a solid proppant core particle that issubstantially covered with a substantially cured, substantiallyhomogeneous coating that comprises the reaction product of a curablecoating mixture that comprises (a) at least one isocyanate component and(b) at least one monofunctional alcohol, amine or amide, whereby thecoated proppant solid is capable of forming particle-to-particle bondsat elevated temperature and pressure.
 11. A proppant according to claim10 wherein said coating mixture further comprises a monofunctionalalcohol.
 12. A proppant according to claim 10 wherein said coatingmixture further comprises a monofunctional amine compound.
 13. Aproppant according to claim 10 wherein said curable coating mixturecomprises a monofunctional amide.
 14. A substantially cured, coated,proppant solid comprising a solid proppant core particle that issubstantially covered with a substantially cured, substantiallyhomogeneous coating that comprises a reaction product of a coatingmixture that comprises: (a) an isocyanate-functional component, (b) anamine-based or polyalkyeneglycol polyol component curing agent and (c)an amine or metallic co-catalyst, whereby said substantially curedcoating is capable of forming particle-to-particle bonds that reduceproppant flowback under downhole conditions.
 15. A proppant solidaccording to claim 14 wherein said coating mixture further comprises:(d) a curing agent comprising at least one monofunctional alcohol,monofunctional amine or monofunctional amide.
 16. A free-flowing, coatedproppant that acts as a pre-cured proppant for handling and crushresistance as well as acting like a partially cured proppant capable offorming particle-to-particle bonds at elevated temperature and pressurewherein the coating is made from a curable coating mixture thatcomprises (a) an isocyanate-functional component and (b) apolyol-functional component, wherein said coating mixture exhibits aweight ratio of said isocyanate-functional component that is within therange from about 100-400 wt % of said polyol-functional component.
 17. Aproppant according to claim 16 wherein said isocyanate-functionalcomponent is within the range from about 100-175 wt % of saidpolyol-functional component.
 18. A proppant according to claim 16wherein said isocyanate-functional component is within the range fromabout 200-400 wt % of said polyol-functional component.
 19. A proppantaccording to claim 16 wherein said coated proppant is substantiallycovered with a substantially cured, substantially homogeneous coating ofsaid curable coating mixture in which said isocyanate-functionalcomponent comprises an oligomer based on methylenediphenyl diisocyanate,said polyol-functional component comprises an aliphatic polyol, andfurther comprises at least one of a monofunctional alcohol, amonofunctional amine or a monofunctional amide.
 20. A proppant accordingto claim 16 wherein said coating mixture further comprises a metallicco-catalyst.
 21. A proppant according to claim 15 wherein said proppanthas been substantially coated with an anticaking agent.
 22. A proppantaccording to claim 21 wherein said coated proppant has beensubstantially coated with an amorphous silica anticaking agent.
 23. Aproppant according to claim 16 whereby an interparticle bond strengthidentified from a standard unconfined compressive strength test issubstantially unaffected by prior exposure for at least two hours at anelevated temperature of at least 200° F. before application of anapplied crack closure stress and corresponding particle-particle contactcaused by such closure stress.
 24. A coated proppant solid whereby aninterparticle bond strength identified from a standard unconfinedcompressive strength test is substantially unaffected by prior exposurefor at least two hours at an elevated temperature of at least 200° F.before application of an applied crack closure stress and correspondingparticle-particle contact caused by such closure stress.
 25. A coatedproppant for low temperature wells that exhibits interparticle bondingwithout the use of an activator plasticizer in an unconfined compressivestrength test simulating downhole conditions in a low temperature well.